Porous membranes have been utilized for a variety of purposes such as ultrafiltration and reverse osmosis. They function as a screen or sieve and retain on their surface particles larger than their pore diameter.
There are many potential applications for porous membranes. For example, the smoking of tobacco has been widespread for many years and has resulted in substantial concerns regarding the health hazards caused by toxic components of tobacco smoke. Conventional fibrous filters have not been very satisfactory in reducing the amount of tar, carbon monoxide and other components of smoke.
Porous membranes have also been used to treat waste water, purify sewage, separate solvents from solutions containing particulates and in the desalination of sea water.
A variety of techniques have been utilized to prepare porous membrane filters. U.S. Pat. No. 3,100,721 describes a process for making a microporous film from nylons, polyesters and vinyl polymers by coating a polymer dispersion onto a cellophane sheet, treating the coating with water, drying the coating and stripping it from the cellophane sheet.
U.S. Pat. Nos. 3,642,668, 4,203,847 and 4,203,848 describe the production of a porous polyvinylidene fluoride film by coating a polymer solution on a substrate that is immersed in a leaching bath.